This invention relates to a frequency detection circuit, and in particular to a state machine with hysteresis for detecting frequency of an input signal.
Environment protection has been a major issue to all industries throughout the world in recent years. The energy saving is one of the measures of the environment protection. To achieve the goal of energy saving, producing monitors with energy saving capability is the major goal of most monitor manufactures in these days.
Not long ago, Video Equipment Standard Association (VESA) has adopted a standard for video interface control signals. For example, in the VESA standard, video horizontal synchronization (Hsync) signal is treated as existing when its frequency is larger than 10 KHz, and is treated as non-existing when its frequency is less than 10 Hz. Also, in the VESA standard, video vertical synchronization signal (Vsync) is treated as existing when its frequency is larger than 20 Hz and is treated as non-existing when its frequency is less than 10 Hz. This standard for Hsync or Vsync, such as shown in FIG. 1 for Vsync signal, requires a hysteresis on the status, eg. existing or nonexisting, of Vsync or Hsync signal with respect to the frequency of Vsync or Hsync signal. In other words, when the frequency of Vsync is determined to be larger than 20 Hz, status of Vsync is existing (logic 1) and when the frequency of Vsync is determined to be less than 10 Hz, which is different from and less than 20 Hz, the status of Vsync is non-existing (logic 0 ).
When the Hsync or Vsync signal is sensed to be non-existing by the control circuits within the monitor, the control circuits shut down the power for most circuits or parts of the monitor, forcing the monitor to enter a sleeping mode, in order to save a substantial amount of energy which is wasted without this energy saving capability.
Typically, a frequency detection linear circuit in accordance with the conventional approach has an integration circuit for generating a voltage signal by integrating the input signal. Then a comparator is employed to compare the voltage signal with a reference voltage in order to determine whether the frequency of the input signal exceeds a predetermined high threshold value or falls below a predetermined low threshold value. The conventional approach as recited above needs a substantial amounts of discrete circuits. Furthermore, a linear circuit with hysteresis is very complex to design and cost is rather high.
Alternatively when a frequency detection digital circuit is adopted, a counter is used to calculate the number of pulses within a predetermined amount of time period. After the number of pulses is obtained, some logic circuits are required to meet the requirement of the hysteresis phenomenon.